1. Field of the Invention
The invention relates to the production of microsensors machined in silicon, in particular accelerometers for applications of assisting with navigation in aircraft, and pressure sensors.
2. Discussion of the Background
Various embodiments of such sensors have already been proposed in the art, and in particular sensors produced by superposition of three silicon plates assembled together by bonding or welding, the plates being machined partly before assembly and partly after assembly. The central plate has directly active mechanical elements, that is to say on which the acceleration is exerted in the case of an accelerometer, the pressure in the case of a pressure sensor; it also has electrical elements needed for its operation. The upper and lower plates frame the central plate and, where appropriate, carry the electrical elements needed for its operation, in addition to the electrical elements of the central plate (in particular electrical connections).
One of the main problems which are posed in the fabrication of these sensors is the etching of certain elements of the central plate, and most particularly elements which require high micromachining precision because of their role in the sensor.
This is the case, in particular, with the moving suspension elements of the microsensor, that is to say the mechanical elements which undergo movements or stresses as a function of the parameters which it is desired to measure (accelerations, pressure etc.).
Typically, in an accelerometer having a sensitive mass attached to a fixed framework by suspension elements and a resonant beam, the problem resides in the etching of the suspension elements and of the resonant beam. Similarly, in a pressure sensor having a thin plate experiencing a pressure and transmitting its deformations to the ends of a resonant beam, the problem resides above all in the etching of the resonant beam and of the elements through which it is connected to the thin plate.
In order to obtain sufficient etching precision in terms of the widths and thicknesses of these beams, it is currently necessary to resort to complex silicon-plate structures, with silicon oxide layers integrated in the thickness of the plate (structures with multiple epitaxy or structures with implanted oxide layers), which is expensive. Otherwise, the etching is not precise enough, although the performance of the sensor does in fact depend on the etching precision. It has been found, for example, that the precision with which the beam is etched is not sufficient when the beam is etched by chemical attack on its two faces through a mask deposited on one face.
Lastly, in general, the machining of the elements of the central plate is made particularly difficult because, further to the etching of the active elements, it is necessary to provide other etches, including very deep etches which may extend as far as full separation of an active central area from a peripheral frame, these deep etches having to be carried out without compromising the partial etches of the active elements. The result of this is that the fabrication process has to take a large number of constraints into account, and the object of the present invention is to propose to carry out certain fabrication steps in an advantageous way while being compatible with the many other steps which need to be performed in order to obtain an operational sensor.